This invention describes a new and useful method for removing lignin from lignocellulosics to produce cellulosic pulps for use in papermaking, bleached pulps, regenerated cellulosic products and pulps having increased food value due to greater carbohydrate digestibility as a result of lignin removal from a lignocellulosic material and any other product in which the removal of lignin from a lignocellulosic is beneficial.
Treatment of lignocellulosics with monoperoxysulfuric acid (H.sub.2 SO.sub.5) and its salts has been described in U.S. Pat. Nos., 4,404,061 and 4,475,984. The claims of the former patent are focused on bleaching of pulps and the latter on a pretreatment of wood chips prior to alkaline pulping. Delignification of wood or other lignocellulosics with monoperoxysulfuric acid or its salts is not disclosed or claimed.
In both U.S. patents, a preferred pH range of 3.0-12.0 is disclosed, the pH being adjusted by addition of alkali or acid to the monoperoxysulfuric acid solution. A pH range of 2.0-12.0 is claimed. For bleaching, the amount of monoperoxysulfuric acid on oven dried pulp is given as being in the range of 0.5-5% as KHSO.sub.5 at 10% consistency. Under these conditions the weight percent of monoperoxysulfate anion is 0.04-0.4% of the weight of bleach liquor.
The use of monoperoxysulfuric acid as a pretreatment for alkaline pulping in U.S. Pat. No. 4,475,984 reports a KHSO.sub.5 range of 0.35-35% weight based on the weight of wood chips. At the 7.6 to 1 liquor to wood ratio disclosed in the Patent, the weight percent of monoperoxysulfate anion is 0.034-3.4% of the pretreatment liquor.
In Khim. Drev. (Riga) 9: 109-117 (1971) monoperoxysulfuric acid (Caros acid) in concentrated sulfuric acid (20-50% is used to effectively delignify spruce wood both in terms of yield of cellulose residue and quantity of lignin removed. Examining the data from this publication one sees the preferred condition is 20-50 percent sulfuric acid, and 0.2 grams active oxygen per gram of wood, the active oxygen being one of the oxygen atoms in the monoperoxysulfuric acid molecule. It was noted that detectable concentrations of H.sub.2 SO.sub.5 were present only when sulfuric acid concentrations were greater than 20% in the system sulfuric acid, ammonium persulphate. Twenty percent sulfuric acid corresponded to the lowest concentration at which delignification of the wood began and 50% acid was the optimum concentration for delignification. It was not considered to generate the Caros acid in concentrated sulfuric acid, then dilute, and use the diluted solution for delignification of lignocellulosics.